I have some Plant tabs, but I'm not sure how to put them in my tank. The box says, "Deposit 1 Plantabb (either whole or crushed) for every 5 gallons of water every 10 days". Should I just add them to the water or push them into the substrate??

Just follow the directions on the container. Fertilizers are usually water soluble. There are some on the market that have you push the tablets into the substrate near the plant root. Others are dissolved in the water. I have found it easier to use the ones you dissolve by crushing them up and mixing them in a container outside of the aquarium then pouring the mixture into the tank. Either way, you may have a cloudy tank for a day or so.